1. Field of the Invention
This invention relates to methods of inspecting wafers for manufacturing light emitting elements, and more precisely, methods of inspecting wafers for manufacturing light emitting elements with the double-hetero structure formed on them.
2. Prior Art
Light emitting elements such as light emitting diodes are usually obtained by layering a plurality of semiconductor layers on a semiconductor substrate to prepare a wafer for manufacturing light emitting elements with a pn junction(s), and making it into an element.
FIG. 11 is an example of a light emitting diode. It shows a cross section of a GaAlAs wafer for manufacturing light emitting elements which has the double-hetero (DB) structure. In this figure, a p-type clad layer 11 consisting of a mixed crystal compound semiconductor represented by Ga1-zAlzAs (where 0&lt;z&lt;1), a p-type active layer 12 consisting of a mixed crystal compound semiconductor represented by Ga1-xAlxAs (where 0&lt;x&lt;0.7), and a n-type clad layer 13 consisting of a mixed crystal compound semiconductor represented by Ga1-yAlyAs (where 0&lt;y&lt;1) are formed one after another on the semiconductor substrate 10 consisting of GaAs. Here, the numbers x, y and z satisfy relationships represented by x&lt;y and x&lt;z.
Within the p-type clad layer 11 in the proximity of said p-type active layer 12, a local formation of a n-type inverted layer 14, as shown in FIG. 11, may occur due to inadvertent introduction of n-type impurities and such. When this n-type inverted layer 14 is formed, the structure of the light emitting element is broken, and that portion becomes defective. On the other hand, within the n-type clad layer 13 in the proximity of said p-type active layer 12, a local formation of a p-type inverted layer 9, as shown in the figure, may occur due to inadvertent introduction of p-type impurities and such. When this p-type inverted layer 9 is formed, the structure of the light emitting element is broken and that part also becomes defective.
Also, since the p-type active layer 12 is relatively thin, the active layer deficient area 17 which is partially missing the p-type active layer 12, as shown in FIG. 11, may result due to abnormal conditions of epitaxial growth and such. This area also becomes a defect.
When said defects are formed in a wafer for manufacturing light emitting elements, as described above, the light emitting elements manufactured by using the areas which include them will have defective electrical characteristics and light emitting characteristics, and thus they will not be acceptable for use.
Conventionally, wafers for manufacturing light emitting elements have been inspected for defects as described above by means of measurement of their electrical characteristics. The procedure comprises, for example: Attaching electrodes to the manufactured wafer for manufacturing light emitting elements, separating each element by means of dicing, assembling it as a lamp, applying a constant electric current to it, and thus inspecting voltage characteristics. Since this inspection is destructive testing, it is not an inspection for an entire wafer surface, but is rather a partial inspection.
Since the conventional procedure for inspecting wafers for manufacturing light emitting elements, as described above, involves inspecting the electrical characteristics after assembling a certain portion of the wafer for manufacturing light emitting elements as a lamp, it was not possible to detect defective areas at the wafer stage.
Therefore, this method, as a method of inspecting wafers for manufacturing light emitting elements, has problems. It is destructive testing, it does not guarantee the characteristics of the entire wafer surface because it is a partial inspection, and it requires excessive time to obtain the inspection results.